Transmission diffraction element

Abstract

A transmitting diffraction element may include a transparent substrate, and a plurality of convex parts periodically formed on one surface of the transparent substrate, and be configured to diffract incident light to the transparent substrate. Each of the convex parts may include first, second, third, and fourth layers that are stacked on the transparent substrate. The first and third layers may be made of a material having a high refractive index, and the second and fourth layers may be made of a material having a low refractive index that is lower than the high refractive index.

What is claimed is: 
     
       1. A transmitting diffraction element comprising:
 a transparent substrate; and 
 a plurality of convex structures periodically formed on a surface of the transparent substrate, 
 wherein the plurality of convex structures comprises a first layer stacked on the transparent substrate, a second layer stacked on the first layer, a third layer stacked on the second layer, and a fourth layer stacked on the third layer, the first layer and the third layer are made of a first material having a refractive index, the second layer and the fourth layer are made of a second material having a refractive index that is lower than the refractive index of the first material, the third layer has a thickness that is largest amongst thicknesses of the first, second, third and fourth layers, and a diffraction efficiency of a transmitted first-order diffracted light is not less than 98% for p-polarization and s-polarization in a wavelength range of 1525-1575 nm or 1570-1610 nm when (2λ/P−sin θ)>1 is satisfied such that a transmitted second-order or higher diffracted light is not generated, where λ is a wavelength of an incident light, P is a grating period of the plurality of convex structures, and θ is an incident angle of the incident light. 
 
     
     
       2. The transmitting diffraction element as claimed in  claim 1 , wherein the first material is at least one material selected from the group consisting of Si 3 N 4 , TiO 2 , Nb 2 O 5 , and Ta 2 O 5 , and the second material is one material selected from the group consisting of SiO 2  and SiON. 
     
     
       3. The transmitting diffraction element as claimed in claim  1 , wherein the refractive index of the first material is within a range of 1.9 to 2.3 in a wavelength region of the incident light, and the refractive index of the second material is within a range of 1.4 to 1.6 in a wavelength region of the incident light. 
     
     
       4. The transmitting diffraction element as claimed in  claim 3 , wherein a difference between the refractive index of the first material and a refractive index of the second material is 0.5 or greater. 
     
     
       5. The transmitting diffraction element as claimed in  claim 1 , wherein a difference between the refractive index of the first material and a refractive index of the second material is 0.5 or greater. 
     
     
       6. The transmitting diffraction element as claimed in  claim 1 , wherein the refractive index of the second material is 1.5 or less in a wavelength region of the incident light. 
     
     
       7. The transmitting diffraction element as claimed in  claim 1 , wherein a sum of the thicknesses of the first layer and the third layer is greater than a sum of the thicknesses of the second layer and the fourth layer. 
     
     
       8. The transmitting diffraction element as claimed in  claim 1 , further comprising:
 an anti-reflection coating layer formed on a second surface of the transparent substrate on an opposite side to the surface. 
 
     
     
       9. The transmitting diffraction element as claimed in  claim 8 , wherein the anti-reflection coating layer comprises a plurality of layers each made of the first material and a plurality of layers each made of the second material in which the layers of the first and second materials are alternately stacked. 
     
     
       10. The transmitting diffraction element as claimed in  claim 8 , further comprising:
 a warp adjusting layer formed between the transparent substrate and the convex structures, 
 wherein the warp adjusting layer is made of a material having a refractive index with a difference within ±0.1 with respect to a refractive index of the transparent substrate, and the warp adjusting layer has a thickness in a range of 0.5 to 1.5 times a thickness of the anti-reflection coating layer. 
 
     
     
       11. The transmitting diffraction element as claimed in  claim 10 , wherein the substrate is made of silica glass. 
     
     
       12. The transmitting diffraction element as claimed in  claim 1 , wherein a difference between the diffraction efficiency of a transmitted first-order diffracted light for p-polarization and the diffraction efficiency of a transmitted first-order diffracted light for s-polarization is not more than 0.5%. 
     
     
       13. The transmitting diffraction element as claimed in  claim 1 , wherein the diffraction efficiency of a transmitted first-order diffracted light is not less than 98% for p-polarization and s-polarization in a wavelength range of 1500-1590 nm or 1545-1635 nm, when (2λ/P−sin θ)>1 is satisfied such that the transmitted second-order or higher diffracted light is not generated. 
     
     
       14. The transmitting diffraction element as claimed in  claim 1 , wherein the diffraction efficiency of a transmitted first-order diffracted light is not less than 99% for p-polarization and s-polarization in a wavelength range of 1525-1575 nm, when (2λ/P−sin θ)>1 is satisfied such that the transmitted second-order or higher diffracted light is not generated. 
     
     
       15. The transmitting diffraction element as claimed in  claim 1 , wherein the plurality of convex structures is formed on the surface of the transparent substrate such that the first layer of the convex structures is in contact with the transparent substrate. 
     
     
       16. The transmitting diffraction element as claimed in  claim 1 , wherein each of the convex structures has a four-layer structure consisting of the first layer, the second layer, the third layer and the fourth layer. 
     
     
       17. The transmitting diffraction element as claimed in  claim 1 , wherein each of the convex structures has a cross section in a plane perpendicular to the substrate, and the cross section has a trapezoidal shape. 
     
     
       18. The transmitting diffraction element as claimed in  claim 17 , wherein an angle between a tapered side of the trapezoidal shape and a line perpendicular to the substrate is 3°. 
     
     
       19. The transmitting diffraction element as claimed in  claim 1 , wherein the substrate is made of silica glass. 
     
     
       20. The transmitting diffraction element as claimed in  claim 1 , wherein the diffraction efficiency of a transmitted first-order diffracted light is not less than 98.5% for p-polarization and s-polarization in a wavelength range of 1570-1610 nm, when (2λ/P−sin θ)>1 is satisfied such that the transmitted second-order or higher diffracted light is not generated.